DE 43 28 297 C1 discloses a safety switch having a switch head receiving an actuator fixed, for example, on the protective door of a protective device to turn a ratchet wheel. Within the switch housing, a contact plunger is held in contact with the peripheral surface of the ratchet wheel. At an angle of rotation corresponding to the closed position of the protective door, the contact plunger engages a catch depression provided for this purpose on the peripheral surface of the ratchet wheel, and thus, closes a contact pair. In this way, the inserted state of the actuator and, accordingly, the closed position of the protective door can be electrically signaled.
For many applications, for example, for protective hoods on machine tools or the like, in addition it is necessary for the safety switch to make available a tumbler function, i.e., for the actuator to be lockable in the switch head, in particular to be lockable with respect to being routed out of the switch head, and thus for the protective device to be lockable in the closed state. In the prior art safety switch, the contact plunger is held in the catch depression, loaded by spring force or magnetic force, and thus blocks rotary motion of the cam disk.
For some applications, it is necessary, for example, for reasons of fire protection, for instance, when the electrical voltage supply of the machine is turned off in case of a fault, for the protective device to be locked in the closed state, for example, in the closed position of the protective door. This need engenders the problem that in the otherwise de-energized state of the apparatus, for example, on the occasion of cleaning activities on the weekend, in an unintentional or inadvertent closing of the protective device the tumbler device is activated. The protective device is then locked or held then in the undesirable closed state. The requirement for “tumbler in the de-energized state” and simultaneous “prevention of activation of tumbler in the de-energized state” therefore constitutes an apparently insurmountable contradiction for generic apparatus.
DE 10 2004 047 574 A1 discloses a solution to this problem in which a blocking element is mounted to be transversely movable to the tumbler element and in a position fixing the tumbler element positively to engage a recess in the tumbler element. This known solution dictates a larger-scale design of the apparatus.